Radon is a MAJOR concern in SanFrancisco (due to building traditions in the city), and a major health factor. Radon is (in 2 words) a radioactive gas coming up from the earth. It’s a HEAVY gas that will accumulate over time in any enclosed space directly in contact with the earth.

So radon will be found in in-law units kitchen with Granite countertop (from the granite AND from the soil), and it will dangerously be found in any garden level master suite (as stupidly developed over the last decade by flip-contractors who use any space, including basement, as living space) without the sanitary “crawl space” under the living space.

Remedies: limit the amount of raw earth material inside your house, VENTILATE (a 1500sqft kitchen in a 9000sqft mansion would be fine no matter the amount of granite ), whenever possible, do not live in a room whose floor is a slab-over-the-soil type.

if you cannot avoid the ground floor living space (such as the Twister playroom on Shrader), it’s important to seal the floor with a super-high-quality coat of [ask your contrator. epoxy is usually ok].And make sure you install an automated ventilation system with heat recovery so you can evacuate the radon on a daily basis.
( http://en.wikipedia.org/wiki/Heat_recovery_ventilation ). ((that means that the standard HVAC system is not appropriate. ))